1. Field of the Invention
The present invention relates to positive working polyamic acid photoresist compositions and their use as dielectrics, and particularly to improvements in imagewise resolution upon photo image development.
2. Description of the Prior Art
Photoresist compositions, generally, are well-known in the art as coatings comprising a diazoquinone photo sensitizer and a resinous binder. These compositions are coated or deposited onto certain substrates and when exposed to light of proper wave length (irradiated) the compositions are chemically altered in their solubility to certain solvents (developers). This process is known as photoimage development.
Two types of photoresist compositions are known, namely; negative working and positive working photoresist. The negative working resists are compositions which are initially soluble in the developers, but following irradiation become insoluble. Accordingly, by configuring a specific pattern of irradiation, during photoimage development, those areas of the coating exposed to the light will form raised lines which define negative images. Positive working photoresist compositions work in the opposite fashion. That is, the compositions are initially insoluble in alkaline developer and accordingly, upon irradiation the exposed regions dissolve and form indented lines or cavities that define positive images.
The clarity and precision with which these lines are formed, called resolution, is calibrated in microns of geometry.
In the microelectronics industry, it is important to achieve line resolutions as small as possible, preferably one micron or smaller for coating thicknesses within the range of one to two microns. It is also desirable to employ positive working photoresist rather than negative working photoresist in dielectric applications. The recessed lines formed in the coating from the positive resist will better serve dielectric applications than will raised lines formed from negative resist because in dielectric applications, unlike many other applications, the bulk of the coating remains on the wafer during subsequent processing. Additionally, modern techniques for processing semi-conductors call for plasma and sputter etching, ion beam implantation, and the like, which require photoresist compositions having stability at temperatures as high as 300.degree. and higher. When photoresist compositions are employed as dielectric layers, not only is thermal stability essential, but the resist must also maintain good dielectric properties.
Over the years, polyamic acid condensation resins produced from an aromatic dianhydride and an aromatic di-primary amine, such as those described in U.S. Pat. No. 3,179,634, have received widespread attention as resinous binders for photoresist compositions because they are readily converted by heat to thermally stable polyimides. They are resistant to dilute acids and organic solvents and they are heat stable at temperatures in excess of 400.degree. C. However, because of the high solubility of the polyamic acid in alkaline-developer, prior-to conversion to the imide, their use has been restricted for the most part to negative working photoresist compositions.
The earliest of these negative working polyamic acid photoresist was disclosed in U.S. Pat. No. 3,623,870. Therein, a negative working photoresist composition was disclosed comprising a mixture of photosensitive dichromate and the polyamic acid binder. The composition was initially soluble in the developer and upon photoimaging, the dichromate cross-linked the polyamic acid binder causing the exposed areas to become less soluble than the uncross-linked polyamic acid of the unexposed areas. Thus, development was said to proceed via different rates of solubility between imagewise exposed and non-exposed areas. It was found, however, to be difficult, if at all possible, to prevent some attack by the developer solvent on the non-exposed areas; accordingly, the problems associated with obtaining good resolution from polyamic acid systems arose even in negative working polyamic acid photoresist compositions.
More recently, in U.S. Pat. No. 4,451,551, polyamic acids photosensitized with a compound having an amino group and an aromatic azide group were disclosed in a negative working photoresist composition. After photoimage development, the composition was baked at from 150.degree. to 300.degree. C. to produce a heat resistant polyimide that resisted distortions when heated to 400.degree. C. for an hour and was used as dry-etching resistant photoresist. Again, the composition was initially soluble in the developer and the unexposed portions remained soluble, while the irradiated portions were rendered relatively less soluble for the formation of negative patterns.
More recently, in U.S. Pat. No. 4,515,887, a negative working polyamic acid photoresist specifically designed for use as a dielectric was disclosed. Therein, the base resin was produced by a condensation reaction between aromatic dianhydride and a mixture of aromatic diamine plus amine organo terminated polydiorganosiloxane. The resultant silicone-polyamic acid was modified by a mixture of isocyanato organoacrylate which enabled the polymer to be sensitized with an appropriate photo sensitizer such as Michler's ketone or benzophenone. The composition was then spin coated onto the substrate and heated to 100.degree. C. for partial imidization. Upon exposure to ultra-violet light in alkaline developer solution cross-linking occurred capable of insolubilizing the exposed areas and creating a negative patterned dielectric layer on its surface.
Even more recently in U.S. Pat. No. 4,656,116, a negative working polyamic acid photoresist composition was disclosed wherein aromatic tetracarboxylic acid derivatives and aromatic diamines having both ortho-positions relative to the phenylene radical bonded to an imide group of the polymer and substituted by alkyl groups where radiation-crosslinked with organic chromophoric polyazides. The compositions were useful in preparing dielectric layers and for producing printed circuits and integrated circuits.
Several problems associated with the use of negative working polyamic acid photoresists for imagable dielectric layers could be overcome if positive working photoresists were used. In the first place, dielectric applications commonly require that holes be patterned in the existing coating. Hole patterning is most effectively accomplished through the use of a positive resist, in which the exposed areas are removed. In addition, a slight side slope is desired in order to achieve effective metal contacts. Positive resists naturally achieve the necessary sloping sidewalls because the top of the film receives a higher exposure than the bottom of the film, and is therefore slightly more soluble. The slope produced in a negative resist is in the opposite direction from that desired, because solubility is inversely related to exposure. A third advantage of positive resist is especially important in thick films where absorption of the film can greatly decrease sensitivity. Positive resists photobelach--that is, the absorption coefficient decreases upon exposure, enabling more light to reach the lower regions of the film, and increasing exposure efficiency. For these reasons, a positive resist is preferable to a negative resist for dielectric applications.
The earliest attempts to make thermally stable polyamic acid photoresist compositions in a positive working fashion was disclosed in U.S. Pat. No. 4,093,461. Therein, it was attempted to make the polyamic acid condensation resin insoluble in an alkaline developer by admixture with orthoquinone and orthonaphthoquinone diazide photosensitizers. It was believed that sufficient quantities of the diazide sensitizer would render the unexposed areas of the photoresist composition completely insoluble in the aqueous alkaline developing solution because of the hydrophobicity and insolubility of the diazides themselves before photolysis or photoimage development. Hence, a positive image could be formed on the support corresponding to the master pattern of configurated irradiation used during the photoimage development. It was believed that a difference in solvation was required to create the positive working phenomenon and/or complete insolubility in the developer as by use of large quantities of the photosensitizer particularly the abietyl types of diazides.
By use of this different solvation, it was believed that sharp distinctions between imagewise exposed and non-exposed areas during development would occur and thus ensure that only light-struck areas were dissolved in the developer; whereas non-exposed areas would remain insoluble and unaffected in the developer. However, such attempts have had only limited success in that polyimide-based photoresist systems exhibit such a high dissolution rate in alkaline solutions with conventional, commercially available diazide sensitizers, that adequate control over processes to obtain high resolution is not possible.
Attempts to decrease the dissolution rate under the teachings of U.S. Pat. No. 4,093,461, by increasing the concentration of the sensitizer in the photoresist to as high as 50% by weight have been problematic for two reasons: (1) The concomitant increase in optical density of the photoresist inhibits full penetration of the film thickness by the radiation source, and (2) there is a progressive reduction in thermal stability of the photoresist associated with increased concentration of the sensitizer.
More recently, positive working polyamic photoresist compositions have been designed to overcome the problems associated with increased sensitizer concentration.
In European Patent Application No. 224,680, it is disclosed that by reducing the acidity of the polyamic acid by about 10 to about 40% of its original value, the dissolution rate of the initial unexposed photoresist composition in the alkaline developer and the subsequent dissolution rate of the exposed areas could be controlled to provide a tailored development rate within a desired range. This reduction in acidity is achieved by, for example, pre-baking to achieve partial imidization; or partial neutralization with basic organic reagents including the use of blends of the polyamic acid and its ester derivatives or co-polymers of the acid and esters derived from the organic reagents. However, pre-baking to achieve partial imidization is therein taught to be limited in the extent to which it could be employed to reduce acidity because temperatures above about 100.degree. C. as for example, even 120.degree. C., caused loss of photosensitivity by degradation of the diazoquinone photosensitizers. Furthermore, acidity reduction through employment of the basic organic reagents, therein disclosed, tends to corrode the conductors found in integrated circuits. Small mobile impurities, created therefrom, tend to degrade dielectric performance.
Accordingly, in spite of the fact that the microelectronics industry's manufacturing processes are based almost exclusively on positive photoresist techniques, those more desirable thermally stable polyamic acid photoresists which are of any practical utility continue to be negative working. The most prevalent commercially available positive photoresist compositions continue to consist of base resins made from phenol-formaldehyde condensation products such as Novolacs. These Novolac based photoresists, when mixed with standard photosensitizers, become insoluble and thereafter allow exposed areas during irradiation or photodevelopment to become soluble. Novolac melts at temperatures above about 150.degree. C. with the result that such systems are not suitable for the modern technology high temperature applications, particularly as interlayer dielectric coatings for integrated circuits.
It would therefore be a substantial advancement in the art to develop a positive working polyamic acid photoresist composition which was devoid of the above-described drawbacks in the prior art.